Anaerobic digestion of swine manure with crop residues

Jun Zhu, University of Minnesota Extension Engineer

Swine manure is used as feedstock for many digesters currently running around the world to produce biogas (methane) for energy recuperation. Although convenient and feasible, it has been recognized that using swine manure alone may not achieve the most efficient production of biogas due to its inherent deficiency of carbon (i.e., low carbon/nitrogen ratio). According to the Mid-West Plan Service Publication, the carbon/nitrogen (C/N) ratio for swine manure is around 6 to 8, depending upon pig growth stages, which is too low for an anaerobic digester to function efficiently to utilize the nutrients in the manure and maximize the methane yields. A past study revealed that the optimal C/N range (adjusted by adding either urea or glucose to the flask digesters) for swine manure digestion in terms of maximum methane production was from 15.5/1 to 19/1. Similarly, dairy manure has a C/N ratio of 8 and the greatest methane production is achieved when its C/N ratio is adjusted to 25/1 using glucose. Practically, using urea and/or glucose to change the C/N ratio in digester content cannot be an economically sustainable method to facilitate methane generation from large-scale digesters. However, the pioneer work reported by the previous researchers has provided convincing evidence that the productivity of anaerobic digestion process can be enhanced by optimizing the substrate C/N ratio. Therefore, it is worth the effort to research alternative additives to accomplish the C/N ratio modification in the digester feedstock.

Crop residues such as straws and corn stalks are produced in large quantities in both the US and other countries every year, which, due to its organic nature, can be a valuable alternative feedstock for biogas production. The benefits of co-digesting plant materials with animal manure lie in that manure can provide buffering capacity and a wide range of nutrients, while the added plant materials with high carbon content can improve the C/N ratio of the feedstock, thus potentially improving methane yields. Because of its large unexploited benefits for biogas production via anaerobic digestion, crop residues certainly deserve more research attention for being used as a feedstock for co-digestion with swine manure.

What we did

In this project, the viability and feasibility of co-digesting three agricultural residues (without pretreatment), i.e., corn stalks, wheat straw, and oat straw, with swine manure to produce biogas were investigated. The amounts of crop residues added to swine manure were calculated to adjust the C/N ratios of the digester content to 16/1, 20/1, and 25/1. The crop materials were first chopped into small sections and then ground into small particles which were further sieved using 40 mesh sieves (0.422 mm pore size) so that all the tested materials had similar particle sizes. The best C/N ratio was obtained from each crop residue.

What we found

Effect of agricultural residues on biogas productions

According to Figure 1, all additives have significantly increased daily biogas production starting from day 1 at all C/N ratios. For C/N=16/1, the increases in average daily biogas volume produced were 6.97, 6.6, and 3.36 times the control for corn stalks, oat straw, and wheat straw, respectively. Similarly, the biogas volumes were increased for these additives at C/N=20 by 11.36, 8.45, and 6.12 times the control. For C/N=25/1, due to equipment failure, the data for oat straw were lost; however, corn stalks and wheat straw continued to overrun the control by 11.59 and 8.26 folds, respectively. These observations conclude that biogas productivity during anaerobic digestion can be improved by addition of carbon rich crop residues if the amount of added materials is calculated based on the C/N ratio requirement for maximizing the process performance.

C/N = 16:1

C/N = 20:1

C/N = 25:1

Figure 1. Click any image to enlarge.

Performance comparison among the three added materials

As seen in Fig. 1, at the C/N ratio of 16/1, corn stalks reached a higher level of volumetric biogas production beginning day 1 of digestion, as opposed to other treatments, and peaked on day 7 (6.6 L/day) before descending on day 9. In contrast, the digester with oat straw gradually increased biogas production and surpassed corn stalks to become the highest producer on day 7 (7.2 L/day) before heading down on day 9. They both performed significantly better than wheat straw in most sampling days before all three converged on day 22 and 25. The control digester produced the least biogas among all tested with the highest daily volume never exceeding 2 L.

At the C/N ratio of 20/1, it was observed that the biogas productivity was increased for all the added agricultural residues with the maximum daily volumes being 11.55L, 9.0L, and 6.93L for corn stalks, oat straw, and wheat straw, respectively, as compared to 6.6L, 7.2L, and 3.51L for the C/N ratio of 16/1. Corn stalks scored much higher in total biogas volume production than oat and wheat straws up to day 9 and being on par with oat straw on day 11 and 14. From day 19 to the end of experiment, although the total daily biogas volume produced by corn stalks was not statistically higher than either wheat or oat straws, the mean volumes were still outstripping the other two, making corn stalks the most productive additive in improving biogas production among the three materials tested when co-digested with swine manure at a C/N ratio of 20.

Oat straw demonstrates its capability of quickly reaching stable CH4 content in the biogas stream (2 days), which is faster than corn stalks (7 days) and much faster than wheat straw (16 days). The CH4 content in all the treatments including the control was relatively stable once it reached the respective peak levels. Although oat straw produced better results in terms of percent CH4 content than corn stalks in the first two sampling days, there was little statistical difference in net CH4 volume produced between these two treatments over the experimental period because corn stalks had achieved higher biogas productivity in the early sampling days than oat straw.

By and large, both corn stalks and oat straw worked equally well in terms of increasing the CH4 content since day 5 at the C/N ratio of 20/1. However, in the last four sampling days (day 16, 19, 22, and 25), the CH4 content in biogas generated from the corn stalks treated digester was consistently higher than that from the digester treated with oat straw (67.5, 68.1, 69.2, and 68.1% versus 56.7, 59.6, 56.0, and 55.6%), implying that using corn stalks as a co-digestion agent with swine manure could increase the methane content in the biogas by roughly 11% as opposed to using oat straw, which was significant. It is also observed that unlike the scenario in the C/N ratio of 16/1 where there is virtually no statistical difference in CH4 content among all the treatments, a clear advantage of adding corn stalks and oat straw over wheat straw, which virtually is not different than the control, to swine manure anaerobic digestion is statistically present for C/N=20.

In view of the net CH4 volume, due to its high biogas productivity in combination of high percent CH4 content in the biogas, corn stalks stands out as the winner among all three materials experimented with the highest daily volume observed on day 9 (7.65 L). Oat straw also produced significantly higher net volumes of CH4 than wheat straw until day 16 and then fell into the same territory of wheat straw. The highest net CH4 volume for oat straw is 5.28L/day and that for wheat straw is 3.39L/day. Comparing information presented in Fig. 1 clearly indicates the influence of increasing C/N ratio on biogas and methane production and the magnitude of these increases are phenomenal. For instance, by increasing the C/N ratio from 16/1 to 20/1, maximal increases observed in daily biogas volume produced and the net CH4 volume were 3.2 fold for corn stalks and 2.8 fold for both oat and wheat straws.

Continuing to increase C/N ratios for corn stalks and wheat straw from 20/1 to 25/1 does not appear to entail the same level of increase in digestion efficiency and/or productivity in terms of daily biogas volume produced, CH4 content in biogas, and net CH4 volume as observed from the previous C/N ratio increase. As a matter of fact, the digestion performance at the C/N ratio of 25/1 is not as good as that at the C/N ratio of 20/1. This indicates that there exists an optimal C/N ratio for co-digesting swine manure with carbon rich agricultural residues for biogas production, which turns out to be 20/1 in this study. C/N ratios higher than 20/1 are considered unnecessary and, instead, may result in technical difficulties in digester operation. It was observed during the experiment that for the C/N ratio of 25/1, agitation of digester content became very challenging due to the high solids content associated with the increased volume of solid materials added. In fact, this was the main cause for the failure of the oat straw digesters at this particular C/N ratio.

Effect of C/N ratio on cumulative biogas productions

Table 1 presents the mean values and standard deviations on the cumulative biogas and net CH4 volumes produced during the entire experimental period for all the treatments including the control. When compared with the control, the benefit of adding these agricultural residues to assist in anaerobic digestion of swine manure is obvious with increases up to 11 times in total biogas volumetric production and 16 times in net CH4 volume across all C/N ratios tested. The higher yields of biogas and methane observed from co-digesting swine manure with crop residues than those from digesting swine manure alone can be attributed to the fact that the volatile solids (VS) in crops is more easily degradable than the VS in manure because the easily degradable part in crops has already passed through the digestive track of animals and has been digested. When comparisons are made among the added residues, corn stalks apparently have achieved the highest biogas volume at all C/N ratios, followed by oat straw (although data for C/N=25 are not available) and wheat straw. When examined under each C/N ratio, the average total biogas volume produced by corn stalks more than doubled that produced by wheat straw for the C/N ratio of 16/1. As the C/N ratio increased, the gap between the total gas productions by corn stalks and wheat straw narrowed slightly but was still remarkably large (81.7 vs. 44.0 at C/N=20 and 83.3 vs. 59.4 at C/N=25). A similar trend is also observed between data for oat and wheat straw.

* Different letters indicate statistical difference between data in the same row but different columns at a significance level of α≤0.05.

Conclusion

The results from this study clearly indicate that significant increases in volumetric biogas production can be achieved by adding carbon rich agricultural residues to the co-digestion process with swine manure. Among the three materials tested, corn stalks and oat straw performed better than wheat straw by all measures including total daily biogas volume, percent methane content in biogas, and net methane volume. The C/N ratio of 20/1 was found to be the best in terms of biogas productivity with increases up to 11 folds in cumulative total biogas production and 16 folds in cumulative net methane volume observed for corn stalks, as compared to the control, which is significant and phenomenal.